1. Field of the Invention
This invention relates to the production of hydrocarbons from coal and similar carbonaceous solids and is particularly concerned with processes for the gasification of coal and other carbonaceous solids in fluidized bed reactors.
2. Description of the Prior Art
The advantages of fluidized bed reactors for the gasification of coal and similar carbonaceous solids are well known. Such reactors permit the handling of large volumes of particulate solids on a continuous basis, are capable of accepting feed materials of relatively wide particle size distributions, permit high heat transfer rates, result in substantial temperature uniformity, facilitate the control of reaction conditions, and have other characteristics which generally make them superior to other systems. One disadvantage of such reactors, however, is that fine particles which are present in the feed material or are produced during the gasification reaction tend to be carried overhead with the raw product gas and must be removed prior to further treatment of the gas. This is normally done by first passing the gas stream through one or more cyclone separators or similar devices which remove the larger entrained particles and then scrubbing the gas to take out additional entrained fines.
Studies have shown that the char fines carried overhead with the product gas during fluidized bed coal gasification and similar operations represent a significant part of the total solids present in such systems and, depending upon the particular solid feed material and reactor conditions employed, may correspond to as much as 10 to 25 weight percent of the dry solids fed to the gasifier. These fines often contain from about 50 to 70 weight percent carbon and hence represent a significant loss of feed material. This is reflected by a corresponding reduction in the volume of product gas or the amount of energy recovered from the system. There have been numerous suggestions as to ways in which the fines might be used to reduce these losses. Recycling to the gasifier has been proposed but this is generally impractical. In most instances, the recycled fines tend to move upwardly through the fluidized bed at velocities approaching those of the fluidizing gas and are thus not retained in the bed long enough to permit their conversion. To avoid the continuous recycling and buildup of fine particles in the system, it is generally recommended that the fines be burned as an auxiliary fuel or otherwise utilized outside the gasifier. This reduces the overall efficiency of the process and has other disadvantages.